BRITISH military forces were committed to both Bosnia and Iraq last night by John Major and senior Cabinet colleagues after a six-hour meeting at Downing Street.

Up to 1,800 soldiers will be sent to protect UN aid convoys in Bosnia. The RAF will provide air cover if necessary.

The British troops - likely to be armoured infantry, sappers, and bomb disposal experts - will serve in UN blue helmets to emphasise their humanitarian relief role. The Prime Minister gave an assurance that the troops would not be asked to 'fight to Armageddon and back'.

The decision to deploy a

battalion-strength force of ground troops marked a change of policy by Britain, which earlier ruled out any substantial troop deployment, and carries clear risks. They will be joined by more forces from other European Community partners, including France.

The Defence and Overseas Policy Committee of the Cabinet rejected three other options, requiring 100,000-300,000 men, which it feared would drag Britain into an all-out war in the Balkans. But continued public pressure has forced it to act.

The direct threat of action against Iraq by Britain, France and the US was approved by the Cabinet. An air exclusion zone is to be declared in southern Iraq by the Allies. They will hand an ultimatum to the Iraqi ambassador at the UN warning that Iraqi jets will be shot down if they continue to fly over southern Iraq.

Six RAF Tornado reconnaissance jets will be sent to Saudi Arabia in the next three days with ground staff and a refuelling aircraft. A similar force will be dispatched by France, but the main force will be provided by the US which has sent a battle group with an aircraft carrier to the area. It is likely that the 'no- fly zone' will be implemented by the weekend.

The United Nations was not consulted before yesterday's decision, nor does any Security Council resolution authorise the actions being carried out in the name of besieged Marsh Arabs.

Instead the allies were acting under a controversial principle of international law permitting military intervention in cases of grave humanitarian abuse. Military intervention a year ago to protect Kurdish refugees was carried out under the same principle. Humanitarian abuses were not enough to provoke the US and its allies to create a similar zone in Bosnia, however, despite appeals from the Sarajevo government.

The Cabinet also approved a warning of further force, directed at Baghdad, if President Saddam Hussein continues to flout UN resolutions requiring inspection and destruction of his arsenal of Scud missiles and nuclear or chemical warfare capability. That could include missile or bombing raids on Iraqi military targets and ministries to which the UN teams have been denied access, but the Prime Minister's office would give no further details.

Mr Major said: 'What we have said to the Iraqi authorities is that we are perfectly clear they are engaged in systematic repression of the Shias in the south and that has got to stop.

'We will monitor the whole area from the air and instruct the Iraqis not to fly in that area. If they do, they will be attacked.'

Although the deployment of troops in Bosnia is an escalation of the British commitment, the Cabinet regarded its approval of force as the minimum required to enforce the UN resolutions on the former Yugoslavia and Iraq.

The Cabinet, briefed by Field Marshal Sir Richard Vincent, Chief of Defence Staff, decided the UN aid convoys in Bosnia had to be reinforced after an attempt to blow one up on Monday with five mines and a trip wire. They were told the number of convoys had to be stepped up to avoid a worse disaster as winter sets in with 1.25 million people facing starvation.

Ministers considered three other options for Bosnia: providing support to force convoys through against light opposition, against heavy opposition, and the creation of security corridors to allow convoys to pass unhindered. They rejected all three because of the heavy troop commitments required and the risk of escalation. One senior source said it could have led to the UN being forced to retire, leaving Britons fighting a full-scale war.

The committee, which included Douglas Hurd, the Foreign Secretary, Douglas Hogg, the Minister of State at the Foreign Office who visited Bosnia, and Malcolm Rifkind, Secretary of State for Defence, appeared to delay the announcement until it could be co-ordinated with the US and France.

The Prime Minister's office made it clear British troops were not being deployed in Bosnia to intervene in the fighting between the Serbs and the Muslims. Britain and its European partners involved in the relief convoys are seeking to avoid the troops coming under fire from either side.

There is a clear risk that the UN flags will not stop sniper fire or further attempts to blow up the convoys. The British troops will include explosives experts to deal with such a threat.

Mr Major told colleagues Britain could not take sides in the conflict. 'There are no heroes and no villains - just a lot of fighting and a lot of people losing their lives,' said a source. The London conference next Wednesday will try to find a peaceful solution and stop the Serbian policy of 'ethnic cleansing' which Mr Major regards as an act against humanity.